Police said the incident in Dewsbury happened at 6.15pm on Thursday, July 7.
The man burst into a convenience store on Old Bank Road, Earlsheaton with a knife and threatened a member of staff.
He then left the store without any cash and escaped down a nearby ginnel in the direction of Wakefield Road.
DC Seb Morgan, from Kirklees CID, said: “This was an extremely terrifying attack for the person involved, we are appealing for anybody who may have witnessed the incident or has any information.”
The suspect is around 25-years-old, 6ft tall, white and thin.
Anyone with information should contact Kirklees CID on 101.